Strained silicon on insulator from film transfer and relaxation by hydrogen implantation

ABSTRACT

Transistors fabricated on SSOI (Strained Silicon On Insulator) substrate, which comprises a strained silicon layer disposed directly on an insulator layer, have enhanced device performance due to the strain-induced band modification of the strained silicon device channel and the limited silicon volume because of the insulator layer. The present invention discloses SSOI substrate fabrication processes comprising various novel approaches. One is the use of a thin relaxed SiGe layer as the strain-induced seed layer to facilitate integration and reduce processing cost. Another is the formation of split implant microcracks deep in the silicon substrate to reduce the number of threading dislocations reaching the strained silicon layer. And lastly is a two step annealing/thinning process for the strained silicon/SiGe multilayer film transfer without blister or flaking formation.

RELATED APPLICATIONS

This is a Division of U.S. patent application Ser. No. 10/755,615 forStrained Silicon on Insulator from Film Transfer and Relaxation byHydrogen Implantation, filed Jan. 12, 2004, now U.S. Pat. No. 6,992,025,granted Jan. 31, 2006.

FIELD OF THE INVENTION

This invention relates to integrated circuit (IC) structures andprocesses, and specifically to an efficient fabrication method of astrained silicon layer on an insulator, yielding a strainedsilicon-on-insulator (SSOI) structure that is useful for high speeddevice fabrication.

BACKGROUND OF THE INVENTION

Transistors fabricated on Silicon-On-Insulator (SOI) substrates havesignificant advantages, such as higher speed, lower power and higherdensity, over transistors fabricated on bulk silicon wafer substrates. ASOI substrate typically consists of a thin surface layer of singlecrystal silicon formed on an insulator layer formed on a bulk siliconwafer. The thin surface silicon layer becomes the silicon channel of thetransistor, and the insulator layer, usually silicon dioxide, iscommonly referred to as the “buried oxide”, or BOX.

SOI wafers improve the transistor performance by reducing the operatingsilicon volume and by isolating the transistors. The thin surfacesilicon layer limits the volume of silicon which needs to be charged toswitch the transistor on and off, and therefore reduces parasiticcapacitance of the transistor, and increases the switching speed. Theinsulator layer isolates the transistor from its neighbors, andtherefore reduces the leakage current and allows the transistor tooperate at lower supply voltages, thus the transistors may be smallerand more densely packed.

SOI substrates are typically fabricated by oxygen implantation into asingle crystal silicon wafer. Recently, a SOI wafer bonding method wasintroduced in which a wafer having a single crystal silicon surface wastightly joined with a wafer having an insulator surface. The compositesubstrate is then polished, or etched, until a single crystal siliconthin film remains on the insulating film.

The SOI wafer bonding method provides a new class of SOI substrates,called SSOI (Strained Silicon-On-Insulator) substrates, where the singlecrystal silicon layer disposed on the insulator substrate is understrain. Transistor devices fabricated on a strained single crystalsilicon substrate have been experimentally demonstrated to have enhanceddevice performance compared to devices fabricated on unstrained siliconsubstrates. The potential performance improvements are due to theelectron and hole mobility enhancements as a result of the introductionof strain-induced band modification of the device channel, resulting inincreased device drive current and transconductance, high circuit speed,low operation voltage and low power consumption.

The strained silicon layer is the result of stress imposed on a siliconlayer deposited on a seed layer, whose lattice constant is differentfrom that of silicon. For larger/smaller seed layer lattice constants,the silicon layer attempts to extend/contract itself to match the seedlayer lattice constant, and experiences biaxial tensile/compressivestress, forming a tensile/compressive strained silicon layer. Forexample, the lattice constant of germanium is about 4.2 percent greaterthan that of silicon, and the lattice constant of a silicon-germaniumalloy is linear with respect to its germanium concentration. As aresult, the lattice constant of a SiGe alloy containing 50% germanium isabout 1.02 times greater than the lattice constant of silicon. Bydepositing an epitaxial silicon layer on a relaxed silicon germanium(SiGe) layer, the epitaxial silicon layer is under tensile strain, andbecomes a single crystal strained silicon layer, suitable for transistordevice strained channel.

Similar to the SOI bonding technique, the fabrication of SSOI substratein general comprises the following steps:

-   1. Preparation of thin strained silicon layer on a silicon substrate    by depositing a strain-induced seed layer such as SiGe, followed by    a strained silicon layer;-   2. splitting ion implantation, usually some form of hydrogen, into    the silicon substrate below the strained silicon layer to generate a    region of subsurface microcracks;-   3. substrate bonding of the silicon substrate to an insulator    substrate to create a composite substrate;-   4. thermal annealing to grow the subsurface microcracks, which    separate the strained silicon layer portion along the hydrogen    implantation region from the silicon substrate; and-   5. surface polishing of the SSOI substrate to achieve a strained    silicon layer smooth surface on the insulator substrate.

A major drawback of prior art SSOI processes is the potential damage tothe strained silicon layer because of the proximity of the cleave planeto the strained silicon layer.

The hydrogen splitting implantation dose of about 4×10¹⁶ should generatesufficient defects and dislocations in the silicon substrate. Althoughthese defects and dislocations are generally located in the siliconsubstrate, some defects and dislocations may propagate into the strainedsilicon layer. For plasma immersion ion implantation process, which isan alternative for low cost hydrogen implantation, the damage zone ismuch broader and thus the strained silicon is more affected. Theextension of defects and dislocations may extend into the matrixmaterial, reaching the strained silicon layer in the absence of anyinterface or boundary.

Also, in the prior art methods to produce a strained silicon layer,conventional practice has been to grow a uniform, or graded, SiGe layerto a few microns to generate sufficient stress wherein misfitdislocations start to form through the SiGe layer to relieve the stressand relax the SiGe layer. However, there are several disadvantages tothe growth of a thick SiGe layer. First, with a few micrometer thickSiGe layer, integration is not easy and is not cost effective. Second,the high defect density in a thick SiGe layer, about to 10⁴ cm⁻² to 10⁷cm⁻², may significantly affect the device performance. More importantly,the thickness of this SiGe layer cannot be easily reduced because of theneed for a high degree of relaxation for strained silicon applications.

Other disadvantage is the high stress involved in prior art SSOIfabrication processes, including the requirement of a long,low-temperature anneal to enhance the bonding strength, high temperatureanneal for splitting the silicon layer, and local non-uniform heating.The formation of blisters and craters, due to high stress relief, in thesurface of a silicon wafer implanted with hydrogen ions after annealingis well-known and remains one of the major issues in SSOI fabricationprocesses. One of the methods to prevent stress built up is to patternthe strained silicon layer, for example, see U.S. Pat. No. 6,767,802,granted Jul. 27, 2004, to Maa et al., for Methods of making relaxedsilicon-germanium on insulator via layer transfer, and U.S. PatentPublication No. 2005/0070115 of Maa et al., published Mar. 31, 2005, forMethod of making relaxed silicon-germanium on insulator via layertransfer with stress reduction, which are owned by the owner of thisApplication, and which are hereby incorporated by reference.

SUMMARY OF THE INVENTION

The present invention discloses a method to fabricate a SSOI (StrainedSilicon-On-Insulator) substrate in which a strained silicon layer liesdirectly on an insulator substrate. The present invention SSOIfabrication process addresses various disadvantages of the prior artprocesses and comprises three novel approaches:

-   1. Using a thin relaxed SiGe layer as the strain-induced layer. An    additional hydrogen implantation and anneal step is employed to    foster the relaxation of the SiGe seed layer. Thus the deposited    SiGe layer can be in the range of 250 nm to 350 nm, which is much    less than the thickness of a few microns reported previously by    other inventors. The present invention thin relaxed SiGe layer    process offers better integration and is more cost effective.-   2. In the wafer splitting step using hydrogen ion implantation,    minimization of defects and dislocation in the strained silicon    layer by the formation of subsurface microcracks occur far from the    strained silicon layer and under the strain-induced layer. By    implanting hydrogen ion far into the silicon substrate well below    the SiGe layer, the threading dislocations do not propagate easily    to the topmost strained silicon layer due to the interface of the    substrate silicon and the SiGe layer.-   3. A two step annealing/thinning method for the fabrication of SSOI    film transfer. Taking advantage of substrate thinning processes that    require much less bonding force of the transfer film on the    underlying substrate, a two step annealing/thinning method for SSOI    fabrication process is successfully developed to reduce blister or    flaking formation of the transfer film. The two step    annealing/thinning method basically comprises a first anneal at low    temperature for wafer splitting, followed by a dry etch process to    reduce the thickness of transfer film without loss of adhesion,    before subjected the composite substrate to a second anneal at high    temperature and a final CMP or selective wet etch.

The present invention SSOI fabrication process can further be adaptedfor silicon compounds such as carbon doped silicon or carbon dopedsilicon germanium, or compound semiconductor substrates such as GaAs orInP, or a mix-matched between silicon, silicon compounds and compoundsemiconductor materials.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the method of the invention.

FIGS. 2-1 to 2-10 depict steps in the method of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The novelties of the present invention SSOI fabrication process include:(1) using a thin relaxed SiGe layer as the strain-induced layer; (2)formation of subsurface microcracks far from the strained silicon layer;and (3) film transfer process without blister or flaking formation.

Using a Thin Relaxed SiGe Layer as the Strain-Induced Layer.

The present invention discloses a method to prepare a thin relaxed SiGelayer of less than 500 nm thickness, and preferably about 250 nm to 350nm with either graded composition or fixed composition, with preferablygreater than 20% germanium concentration. The method comprises the stepsof depositing a thin SiGe layer, subjecting the SiGe layer to arelaxation implantation of ions, and annealing the implanted SiGe layerto convert the implanted as-deposited compressive SiGe layer to arelaxed SiGe layer.

Since the SiGe is deposited on a silicon substrate, the as-depositedSiGe will be compressively strained by following the lattice constant ofthe underlying silicon substrate. The relaxation hydrogen ion implantcan generate defects and dislocations at the silicon region below SiGelayer, and the anneal step can induce the relaxation of the thin SiGe.The relaxation implant dose is much less than the splitting implantdose, with the objective being the creation of misfit dislocations andnot subsurface microcracks.

The disclosed implanted relaxation method can utilize various ionspecies such as ionized atomic hydrogen (H⁺), ionized molecular hydrogen(H₂ ⁺), helium, boron, silicon, argon or any combinations thereof.Ionized atomic hydrogen (H⁺) is commonly used but this implantationprocess is expensive due to the long implantation time. In the presentinvention, the implanted hydrogen relaxation is preferably by molecularhydrogen ions (H₂ ⁺), as disclosed in Maa et al., U.S. Pat. No.6,562,703, entitled Molecular hydrogen implantation method for forming arelaxed silicon germanium layer with high germanium content, granted May13, 2003, hereby incorporated by reference. The employment of singlyionized molecular hydrogen (H₂ ⁺) implantation in the relaxation of SiGefilms results in a reduction in the process time and cost, since thisimplantation process can be done at double the energy and half thecurrent.

Basically, the method comprises the deposition of a layer ofsingle-crystal SiGe overlying a silicon buffer layer with the thicknessof the SiGe layer in the range of 100 nm to 500 nm, together with theimplantation with ionized molecular hydrogen (H₂ ⁺) in a projected rangeof approximately 10 nm to 30 nm into the underlying silicon bufferlayer, and then annealing the implanted layer to form a thin relaxedSiGe layer. The presence of the silicon buffer layer serves to supply afresh silicon surface for SiGe growth, but is not absolutely needed.

The use of singly ionized molecular hydrogen (H₂ ⁺) to relax strainedSiGe films was demonstrated in a series of experiments. Epitaxial SiGefilms of approximately 300 nm were deposited, having graded germaniumprofile, varying linearly from approximately 20% at the SiGe/siliconinterface to approximately 30% at the wafer surface. The as-depositedfilms were strained to be lattice-matched to the silicon substrates. TheSiGe films were then implanted with 1×10¹⁶ cm⁻² to 4.5×10¹⁶ cm⁻²H₂ ⁺ions at energies from 58 keV to 66 keV and 155 keV, and annealed at 650°C. to 800° C. for nine to thirty minutes in an argon ambient atmosphere.

Nomarski microscopy images of the SiGe surfaces as well as x-raydiffraction (XRD) reciprocal space maps near the Si(224) substrate peakwere observed. If the implant is too shallow, the SiGe film has a highdegree of lattice relaxation, but the lattice planes are notwell-oriented (giving a broad XRD peak) and the surface becomes rough.With higher implant energy (˜60 keV) the SiGe film still has a latticerelaxation of more than 80%, but the surface roughness decreasesconsiderably. At even higher implant energies (˜155 keV) the filmrelaxation decreases significantly (˜1%) and unchanged even at muchlonger anneal time, but the surface is very smooth.

The major effect of the relaxation implant is the H₂ ⁺ implant depth. Athigh energy, the defect zone is far down in the silicon region,therefore the source of forming misfit dislocation is too far from theupper SiGe region, and thus almost no relaxation is detected. At lowenergy, the defect zone is within the SiGe region, thus a highrelaxation is achieved, but with a trade off in surface roughness.Therefore by adjusting implant energy and dose, a compromise was found,giving sufficient lattice relaxation while maintaining good crystallinequality and a smooth surface. The preferred projected range of theimplanted ions is in the vicinity of the SiGe/silicon substrateinterface, and more preferably within 10 to 30 nm into the siliconsubstrate. Furthermore, the surface roughness due to the surfaceundulation occurred during SiGe relaxation can be eliminated by a postrelaxation polishing such as CMP (Chemical Mechanical Polishing). Therelaxed SiGe shows a consistent reliable polishing rate as long as thesurface is free from native oxide layer. An additional surface cleaningstep after the CMP can be performed, using a modified SC-1(H₂O:H₂O₂:NH₄OH=5:1:1 solution and its variants) to minimize etching ofSiGe, followed by a standard SC-2 (H₂O:H₂O₂:HCl=5:1:1 solution and itsvariants) clean. Thus the preferred process is to use low energy toachieve high relaxation, and then subjected the SiGe film to a CMPprocess to reduce the film roughness.

The novelty of the method of depositing a thin relaxed SiGe is employedin the present invention for the fabrication process of SSOI substrate.By using a thinner SiGe seed layer, various problems of SSOI fabricationprocess relating to the thick SiGe seed layer mentioned above areeliminated or significantly reduced.

The first embodiment of the present invention SSOI fabrication processemploys the fabrication process of a thin relaxed SiGe seed layer andcomprises the steps of:

-   1. preparing a silicon substrate;-   2. preparing a thin relaxed SiGe seed layer by ion implantation;-   3. depositing a SiGe layer overlying the silicon substrate;-   4. implanting ions into the SiGe layer;-   5. annealing to convert the SiGe layer to a relaxed SiGe layer;-   6. Polishing the SiGe layer surface and performing a post polishing    clean;-   7. depositing an epitaxial silicon layer on the relaxed SiGe layer,    thereby generating a strained silicon layer;-   8. transferring the epitaxial silicon/SiGe multilayer to an    insulator layer;-   9. implanting ions into the epitaxial silicon/SiGe multilayer to    generate a defect zone under the epitaxial silicon/SiGe interface;-   10. bonding the epitaxial silicon layer surface of silicon substrate    to an insulator layer on a second substrate to form a bonded    composite substrate; and-   11. thermally annealing the composite substrate to split the    epitaxial silicon/SiGe multilayer along the defect zone.

The ranges of processes for achieving thin relaxed SiGe by the ionimplantation method are that the SiGe concentration is preferablygreater than 20% germanium concentration; the SiGe can be graded orconstant germanium concentration; the SiGe layer thickness is in therange of 100 nm to 500 nm, preferably between 250 nm to 350 nm; the SiGedeposition temperature is preferably in the range of 400° C. to 600° C.;the implantation dosage of ions is in the range of 2×10¹⁴ cm⁻² to 2×10¹⁶cm⁻² and with an energy in the range of 10 keV to 120 keV; theimplantation range is in the vicinity of the interface of the SiGe layerand the silicon substrate, and preferably about 10 nm to 30 nm into theunderlying silicon substrate; and the annealing process is at atemperature in the range of 250° C. to 1000° C., for a period of time inthe range of 0.1 minute to four hours, or a two step annealing processof a low temperature anneal step (about 250° C., for 10 minutes)following by high-temperature annealing step (about 650° C. to 1000° C.for 0.1-30 minutes); and the thickness of the epitaxial silicon layer isbetween 10 nm to 100 nm.

Within these process ranges, the method produces a thin (100 nm to 500nm) relaxed, smooth SiGe film with high germanium content of greaterthan 20% to serve as a strained-induced seed layer for silicon layer.Further, the H₂ ⁺ can be implanted alone, or with boron, helium,silicon, or other species. Furthermore, the traditional technique ofsingle energy beamline implantation or the emerging technique of plasmaimmersion ion implantation can be used, with various ion species such asionized molecular hydrogen (H₂ ⁺), ionized atomic hydrogen (H⁺), helium,boron, silicon, argon or any combinations thereof.

The Formation of Subsurface Microcracks Under the Strain-Induced Layer

Prior art splitting ion implantation processes are typically performedat an energy of between 50 keV to 150 keV, normally 90 keV, with a doseof 2×10¹⁶ cm⁻² to 10¹⁷ cm⁻², which provides a range of implantation isdepth of roughly 0.5 μm to 1 μm. The major drawback of this process isthe closeness of the cleave plane to the strained silicon layer, andtherefore, although the damage is mostly near the projected range, somedefects and dislocations can propagate into the strained silicon layer.With the SiGe seed layer typically in the range of one micron thick, thefact that the split implantation is within the SiGe layer without anyinterface or boundary further facilitates the extension of dislocations,as shown in SiGe-free strained Si on insulator by wafer bonding andlayer transfer by Langdo et al., Applied Physics Letters, Vol. 82,Number 24, Jun. 16, 2003, pages 4256-4258, using split implantationinside the SiGe layer with 75 keV, H₂ ⁺ species, 4×10¹⁶ dose, and 0.35μm range.

The presence of the interface between the silicon substrate and therelaxed SiGe layer is found to be a good barrier for the propagation ofdefects and dislocations. Similar to the relaxation experiments of thinSiGe films, split implantation near the SiGe/silicon interface at 62 keVand far into the silicon substrate at 155 keV show significantdifferences in the propagation of defects and dislocations after anneal.At 155 keV, the defect zone is about almost 500 nm below theSiGe/silicon substrate interface, resulting in negligible defectpropagation reaching the SiGe layer, presumably because the defect zoneis far down in the silicon region. The thick silicon layer is expectedto restrict the pile up of dislocations to its upper section after wafersplitting, and leave the SiGe layer with very little dislocation.

Based on these experiments, the present invention SSOI fabricationprocess discloses a process having the split implantation range targetedat about 300 nm to 500 nm below the bulk silicon/SiGe interface. Bysplit implanting deep into the silicon region, which is far from thestrained silicon layer, the bulk thickness of the silicon layer togetherwith the silicon/SiGe interface help retard the propagation of defectand dislocations, resulting in a higher quality strained silicon channellayer.

The thin relaxed SiGe makes the deep split implantation easier sincesimilar implantation energy and dose used for the prior art thick SiGeare adequate. The split implant is preferably at energy below 300 keV,and more preferably at around 140 keV, and a dose of between 10¹⁶ cm⁻²to 2×10¹⁷ cm⁻², preferably 4×10¹⁶ cm^(−2.)

The deep implantation process also allows the use of PIII (plasmaimmersion ion implantation) which may be an alternative for low costhydrogen implantation since the damage zone is much broader in PIIIplasma implanted silicon. Plasma immersion ion implantation is anemerging technology, which promises high dose implantation at low costand could potentially be used in SSOI fabrication process. In plasmaimmersion the implant depth is controlled by acceleration voltage, withproper equipment modification to increase acceleration voltage, it canalso be used for higher energy implantation. In conventional singleenergy beamline ion implantation, the damage only occurs near theprojected range of the implanted ions, but in hydrogen plasma implantedsilicon, the damage layer is quite broad, due to different ion speciesfrom the plasma implanted to different depth. By introducing the damagezone well below the bulk silicon/SiGe seed layer interface, plasmaimmersion ion implantation technique can be applied to the fabricationprocess of SSOI with less damage to the topmost strained silicon layer.

The second embodiment of the present invention SSOI fabrication processemploys the fabrication process of deep split implantation and comprisesthe steps of:

-   1. preparing a silicon substrate;-   2. preparing a relaxed SiGe seed layer overlying the silicon    substrate whereby a silicon/SiGe interface is formed;-   3. depositing an epitaxial silicon layer on the relaxed SiGe layer,    thereby generating a strained silicon layer;-   4. implanting ions into the epitaxial silicon/SiGe multilayer to    generate a defect zone below the substrate silicon/SiGe interface    wherein the projected range of the implanted ions is about 100 nm to    500 nm into the silicon substrate;-   5. transferring the epitaxial silicon/SiGe multilayer to an    insulator layer;-   6. bonding the epitaxial silicon layer surface of silicon substrate    to an insulator layer on a second substrate to form a bonded    composite substrate; and-   7. thermally annealing the composite substrate to split the    epitaxial silicon/SiGe multilayer along the defect zone.

The ranges of processes for achieving SSOI substrate by the deepsplitting implantation method are that the implantation dosage of ionsis in the range of 10¹⁶ cm⁻² to 2×10¹⁷ cm⁻², preferably 4×10¹⁶ cm⁻²; andwith an energy less than 300 keV, preferably 140 keV; the splitannealing process is at a temperature in the range of 350° C. to 1000°C. for a period of time in the range of 0.1 minute to four hours, or atwo step annealing process of a low temperature anneal step to split thewafer (about 350° C. to 400° C., for between 30 minutes to four hours)following by high-temperature annealing step (about 450° C. to 1000° C.for between six seconds to one hour); and the thickness of the epitaxialsilicon layer is between 10 nm to 100 nm.

The relaxed SiGe can be prepared by the relaxation implantation methodas disclosed above. Further, the H₂ ⁺ can be implanted alone, or withboron, helium, silicon, or other species. Furthermore, the traditionaltechnique of single energy beamline implantation or the emergingtechnique of plasma immersion ion implantation can be used, with variousion species such as ionized molecular hydrogen (H₂ ⁺), ionized atomichydrogen (H⁺), helium, boron, silicon, argon or any combinationsthereof.

The Multilayer Film Transfer Process Without Blister or FlakingFormation

One of the major issues with SSOI fabrication process is stress,resulting in blister or flaking formation of the transferred film. Forexample, in prior art deposition of relaxed SiGe and strained silicon,because of the SiGe stress and low bonding energy, the SiGe layer tendsto buckle and wrinkle to relieve the stress.

By understanding the stress issues in SSOI fabrication process, we areable to design and verify the process that would significantly reduce oreliminate the blister or flaking formation to successfully fabricategood SSOI substrates. Specifically, the substrate bonding processrequires high temperature anneal to improve the adhesion, and the higherthe annealing temperature is, the better the adhesion will be. Theability to sustain high temperature anneal is related to the thicknessof the bonded film, and the thinner the transfer film is, the better itcould tolerate stress built up resulting from high temperature anneal.Thus thinning the transfer film is essential before the high temperatureanneal. Further the adhesion between the bonded substrate resulted fromthe low temperature anneal is normally not adequate for the conventionalsubstrate thinning methods of polishing (such as Chemical MechanicalPolishing, CMP), or wet etch since these thinning methods requiresignificant bonding strength.

The present invention takes advantage of a substrate thinning process ofdry etch that requires much less adhesion of the transfer film on theunderlying substrate, and discloses a two step annealing/thinning methodfor SSOI fabrication process to reduce blister or flaking formation ofthe transfer film. In general, the method comprises the steps of:

-   1. first annealing to split wafer at low temperature: low enough not    to generate blister and high enough to improve the adhesion. In    general, anneal temperature of less than 550° C., preferably less    than 400° C., and most preferably at about 375° C., is adequate to    prevent formation of wrinkles, blisters and buckling. Annealing at    higher than 650° C. is too high because even though there are no    buckles or wrinkle, the SiGe film exhibits blisters;-   2. first thinning the transfer film by low adhesion etching method,    such as a dry etch process, a reactive ion etching process, an ion    milling process, or a sputter etch process. The transfer film should    be thin enough to sustain the next high temperature anneal, and    still thick enough for device and integration purposes, to have    adequate thickness uniformity and adequate thickness. The main    requirement of this first thinning step is low adhesion etching, and    other requirements, such as etch selectivity and etch uniformity,    are not critical in this first thinning step;-   3. second annealing at high temperature to improve the bonding    strength; and-   4. second thinning the transfer film by high adhesion etching    method, such as CMP or a wet etch. The main requirements of this    second thinning step is etch uniformity and etch selectivity to    achieve device and integration requirements of VLSI processing.

This two step annealing/thinning process can be applied to thicktransfer films, including, but not limiting to, the disclosed processabout split implantation deep into the silicon substrate. Even with thethin SiGe layer, by split implantation deep into the silicon substrate,the multilayer film to be transferred is still thick enough to sustain ahigh temperature anneal needed to improve the substrate bonding. Andwithout the improved substrate bonding, the composite substrate cannotsurvive the polishing or wet etch step. The present invention two stepannealing/thinning process can solve this problem. By annealing at lowtemperature for wafer splitting, the blister formation is prevented andthe bonding is improved enough for a subsequent dry etch process. Afterthe multilayer film is thinned by dry etching, the composite substratecan sustain a high temperature anneal needed to improve the substratebonding for the next polishing and selective wet etch steps.

To eliminate blister formation, the first anneal of wafer transfer orsplitting is preferably carried out at temperature less than 400° C.Typically, it is sufficient to transfer in 60 minutes at 375° C. Fordeeper splitting implant (such as at 140 keV energy), a longer time,such as two hours, is needed. Temperature seems to be the dominantforce, and the addition of a cap layer, such as a thin oxide layer, doesnot seem to reduce the blister problem. Normally CMP is the preferredmethod to remove the surface roughness generated from the splittingprocess, but the film adhesion after the first low temperature anneal isonly marginal to withstand the CMP process and the water rinse cycles,therefore CMP is only applied after the second anneal with temperaturehigher than 650° C. This high temperature anneal could generate blistersin the thick transfer film, and therefore, to avoid blister formation,the film is first thinned down by a dry etch step. The dry etch ofreactive ion etching is a preferred method, but other etching steprequiring low film adhesion could be used, for example ion millingprocess, sputter etch process. The second thinning process preferablycomprises a CMP step followed by a selective wet etch step.

The selective wet etch, such as etching in a SC-1 solution which has aSiGe/silicon high etch selectivity, is the preferred step to remove theSiGe layer on the strained silicon layer. Following the two stepannealing process for wafer splitting and bond strengthening, thestrained silicon in SSOI process is very stable. The SSOI substrate issubjected to a steam oxidation at 800° C. for 30 minutes, and further at900° C. for 30 minutes, XRD results show no degradation of this strainedsilicon layer.

The Third Embodiment of the Invention

The third embodiment of the present invention SSOI fabrication processemploys the two step annealing process and comprises the steps of:

-   1. preparing a silicon substrate;-   2. preparing a relaxed SiGe seed layer overlying the silicon    substrate whereby a silicon/SiGe interface is formed;-   3. depositing an epitaxial silicon layer on the relaxed SiGe layer,    thereby generating a strained silicon layer;-   4. implanting ions into the epitaxial silicon/SiGe multilayer to    generate a defect zone under the epitaxial silicon/SiGe interface;-   5. bonding the epitaxial silicon layer surface of silicon substrate    to an insulator layer on a second substrate to form a bonded    composite substrate;-   6. thermally annealing the composite substrate to split the    epitaxial silicon/SiGe multilayer along the defect zone at    temperature below 400° C.;-   7. dry etching to remove part of the silicon substrate and part of    the SiGe layer;-   8. thermally annealing the composite substrate to improve the    bonding at temperature above 400° C.; and-   9. etching the remaining SiGe layer by a polishing process or by    selective wet etch process or by a combination of polishing and    selective wet etch process. The preferred process is to employ first    a polishing step to smooth the split surface, followed by a    selective wet etch step to remove completely the SiGe layer.    Fabrication Process of the Invention

The above disclosed novelties can be combined or applied separately tofabricate SSOI substrate in which a strained silicon layer lies directlyon an insulator substrate.

FIG. 1 shows the process steps combining all the novelties of theinvention. A silicon substrate is prepared (block 11), and a thin layerof SiGe is deposited on the silicon substrate (block 12). The SiGe layeras deposited is under strained due to the lattice mismatch between SiGeand the silicon substrate. The SiGe layer is then subjected to ahydrogen implantation and anneal (block 13) to generate a relaxed SiGelayer. The depth of the relaxation implantation is in the vicinity ofthe SiGe/silicon interface, and preferably about 10 nm to 30 nm belowthe SiGe/silicon interface. The relaxed SiGe layer is then subjected toa CMP step (block 14) to smooth the SiGe surface, and then a post CMPclean (block 15). A thin epitaxial silicon layer is deposited on therelaxed SiGe layer (block 16). Since the silicon and relaxed SiGe havedifferent lattice constant, the deposited silicon layer is understrained. Hydrogen ions are implanted (block 17) wherein the depth ofhydrogen is below the SiGe/silicon interface by a depth of between about100 nm to 500 nm below the SiGe/silicon interface. The siliconsubstrate, together with the SiGe and the strained silicon layer isreferred herein as a silicon substrate. Another silicon substrate isalso prepared, and thermally oxidized to form a SiO₂ layer on thesubstrate, which is referred to herein as an insulator substrate (block18). The silicon substrate is bonded to the insulator substrate, withthe strained silicon surface in contact with the SiO₂, forming acombined structure, also referred to herein as a composite substrate(block 19). The composite substrate is first cured at 150° C. to 250° C.for four to fourteen hours to enhance the wafer bond before splitting(block 20), then split by a first low temperature annealing step (block21), at temperature below 400° C., producing two modified portions: afirst SSOI portion to be continued processing comprises the insulatorsubstrate, the strained silicon layer, the SiGe layer and a part of thesilicon substrate layer, hereby called the top silicon substrate layer,and a second portion comprises the bottom part of the silicon substrate,to be discarded. The SSOI portion is dry etched to remove the topsilicon substrate layer and a part of the SiGe layer (block 22). Asecond anneal step (block 23) at high temperature (above 400° C.,between 650° C. and 1000° C., and preferably at 650° C.) is performed toincrease the bonding adhesion of the existing bonds. Then a CMP step isperformed to smooth the SiGe layer surface (block 24). Finally, thesubstrate is wet etched selectively to remove the entire SiGe layer(block 25), leaving only the strained silicon layer on an insulatorsubstrate. Detailed pictorials of the process steps are described inFIG. 2.

FIG. 2-1 shows a silicon substrate 30 being prepared, with a layer 32 ofSiGe epitaxially deposited on silicon substrate 30. The germaniumconcentration is above 20%, and preferably in the range of between 20%to 60%, and can be graded or uniform throughout. The SiGe thickness isbetween 100 nm to 500 nm, and preferably between 250 nm and 350 nm. TheSiGe is under biaxial compression strain and no relaxation occurs atthis time. The SiGe deposition is typically carried out at a depositiontemperature between 400° C. and 600° C.

FIG. 2-2 depicts the hydrogen ion 34 implantation and subsequentannealing step to induce relaxation of the deposited SiGe film in whichhydrogen ions, either H⁺ or H₂ ⁺, are implanted through the SiGe film.The dose range is between about 2×10¹⁴ cm⁻² to 2×10¹⁶ cm⁻², and theenergy range is between about 10 keV to 120 keV. The implantation rangeis in the vicinity of the interface of the SiGe layer and the siliconsubstrate, and preferably about 10 nm to 30 nm into the underlyingsilicon substrate, and the annealing process is at a temperature in therange of 350° C. to 1000° C., for a period of time in the range of sixseconds to four hours, or a two step annealing process of alow-temperature anneal step (about 350° C. to 400° C., for between tenminutes to four hours) followed by a high-temperature annealing step(about 450° C. to 1000° C., for between six seconds to one hour). Afterthis implantation and anneal step, the SiGe layer is relaxed, and withthe possibility of a rough surface.

FIG. 2-3 depicts the CMP of the relaxed SiGe surface and post CMP clean.50 nm of SiGe surface removal should be sufficient to obtain a smoothsurface, with 10 nm to 30 nm being the preferred thickness of theremoved SiGe.

FIG. 2-4 depicts the epitaxial silicon layer deposition. Since theunderlayer film is the relaxed SiGe with a lattice constant larger thansilicon, the deposited epitaxial silicon 36 is under tensile strained.The thickness of the deposited epitaxial silicon layer is preferablybetween 10 nm to 100 nm.

FIG. 2-5 depicts the hydrogen ion 38 implantation for wafer splittingwherein hydrogen ions, either H⁺ or H₂ ⁺, are implanted through the SiGefilm. The dose range is between about 10¹⁶ cm⁻² to 5×10 ¹⁷ cm⁻²,preferably 4×10¹⁶ cm⁻² of H₂ ⁺, and the energy is below 300 keV,preferably about 140 keV. The range of hydrogen implantation is wellinto the silicon substrate, from about 300 nm to 500 nm below theSiGe/substrate silicon interface. The implanted hydrogen ions form amicrocrack defect region 40, separating the silicon substrate 30 into abulk silicon substrate 30 a and a top silicon substrate 30 b on eitherside thereof. This structure is referred to herein as the siliconsubstrate 44.

FIG. 2-6 depicts the preparation of an insulator substrate 54, which isanother silicon substrate being thermally oxidized to produce a SiO₂layer 52 over a silicon substrate 50.

FIG. 2-7 depicts the bonding of the silicon substrate 44 to theinsulator substrate 54. Note that the silicon substrate 44 is drawn upside down as compared to previous figure. The composite substrate isformed by direct wafer bonding. In direct wafer bonding, the surfaces ofboth portions are cleaned in a modified SC-1 (H₂O:H₂O₂:NH₄OH=5:1:1)cleaning solution and rinsed in distilled water. After drying, at lessthan 900° C., both surfaces are hydrophilic. The dried,hydrophilic-exposed portions facing one another, are brought intocontact at ambient temperature. The bonding is initialized in a smallarea of the touching wafers by slightly pressing the wafers together.The bonded area quickly spreads over the entire in-contact surfaces,within a few seconds. The bonded substrates can be cured at atemperature of between about 150° C. to 250° C. for between about onehour to fourteen hours to improve the surface bonding.

FIG. 2-8 depicts substrate splitting by first low temperature annealingat a temperature below 400° C., preferably 375° C., for between about 30minutes to four hours, to separate the multilayer of strained siliconlayer 36, relaxed SiGe layer 32 and top silicon substrate layer 30 balong the hydrogen split implant region 40 from the silicon substrateportion 30 a. The splitting results in a SSOI portion 60 to be continuedprocessing to produce the SSOI substrate, and a silicon substrateportion 30 a. The first anneal is performed at low temperature toprevent film blistering. After the split, a dry etch process isperformed to remove the top silicon substrate layer 30 b and a part 32 aof the SiGe layer 32. The transfer films now comprise the strainedsilicon layer 36 and the bottom part of the SiGe layer 32 b. This dryetch step is to reduce the thickness of the transfer layer so that noblister will form in the subsequent bond strengthening high temperatureanneal step. Wet etch or CMP is not desirable at this time due to thepossibility of film lifting from weak bond.

FIG. 2-9 shows the SSOI portion after the dry etch process. The secondannealing step is then performed to increase the bonding strength. Thisstep is essential to ensure that the bond is strong enough to withstandthe following CMP and wet etch step. The anneal is performed at atemperature of between about 500° C. to 900° C. for between about tenminutes to sixty minutes in an inert atmosphere. CMP step is performedto smooth the split surface. The CMP step removes most of the SiGe layer32 b, resulting in a much thinner and very smooth SiGe layer tofacilitate the subsequent selective wet etch process.

FIG. 2-10 depicts a selective wet etch step to remove all remaining SiGelayer without significant damage to the strained silicon layer 36. Thewet etch can be performed in a SC-1 solution. The final SSOI substratecomprises a strained silicon layer 36 lying directly on an insulator 52on a silicon substrate 50.

The fabrication of the disclosed SSOI substrate can be accomplished bysemiconductor fabrication process technology as disclosed above.Although illustrated and described below with reference to certainspecific fabrication processes, the present invention is neverthelessnot intended to be limited to the details shown. The general process ofsemiconductor fabrication has been practiced for many years, and due tothe multitude of different ways of fabricating a device or structure,various modifications may be made in the fabrication process detailswithin the scope and range of the present invention and withoutdeparting from the meaning of the invention.

For example, the present invention discloses a SSOI fabrication processemployed strained silicon structure. However, the disclosed invention isnot limited to just silicon, but the scope of the invention can beapplied to silicon compounds such as silicon germanium, carbon dopedsilicon, carbon doped silicon germanium, compound semiconductorsubstrates such as GaAs or InP, or a mix-matched between silicon,silicon compounds and compound semiconductor materials

Alternatively, the process of causing the wafer splitting can beaccomplished by directly excites the implanted ions or molecular ions bythe high frequency alternating electric or electromagnetic field toincrease the collision frequency, see Lee, U.S. Pat. No. 6,486,008,entitled Manufacturing method of a thin film on a substrate, grantedNov. 26, 2002, which is hereby incorporated by reference.

Also, the introduction of an etch stop layer can be used to facilitatethe smoothing and etching of the SiGe seed layer. By using the etch stoplayer, the need for CMP is avoided and the final device thickness,uniformity, and smoothness are based upon the deposited film instead ofCMP parameters. The etch stop material is chosen so that it can beetched selectively as compared to the substrate material, for example, ahigh doped (p⁺ or n⁺) silicon layer, a silicon-germanium (SiGe) layer, astrained SiGe layer, or a germanium layer can be used as an etch stoplayer. See Srikrishnan, U.S. Pat. No. 5,882,987, entitled Smart-cutprocess for the production of thin semiconductor material films, grantedMar. 16, 1999, which is hereby incorporated by reference.

Further, the substrate bonding process can be performed without thetreatment of hydrophilicity, see Yamagata et al., U.S. Pat. No.6,156,624, Method for production of SOI substrate by pasting and SOIsubstrate, granted Dec. 5, 2000, which is hereby incorporated byreference.

1. A method of forming a strained silicon layer on an insulatorsubstrate comprising providing a silicon substrate; preparing a relaxedSiGe layer directly on the silicon substrate whereby a silicon/SiGeinterface is formed between the silicon substrate and the relaxed SiGelayer; depositing an epitaxial silicon layer directly on the relaxedSiGe layer to form an epitaxial silicon/SiGe multilayer; implantingsplitting ions into the epitaxial silicon/SiGe multilayer to generate adefect zone below the silicon/SiGe interface wherein the projected rangeof the implanted ions is about 100 nm to 500 nm into the siliconsubstrate; and transferring the epitaxial silicon/SiGe multilayer to aninsulator substrate.
 2. A method as in claim 1 wherein the splitting ionimplantation is performed by plasma immersion ion implantation.
 3. Amethod as in claim 1 wherein the implanted splitting ions are molecularhydrogen ions (H₂ ⁺).
 4. A method as in claim 1 wherein the implantedsplitting ions are taken from the group of ions consisting of hydrogenatom ions (H⁺), helium ions, boron ions, silicon ions, argon ions, andany combinations thereof.
 5. A method as in claim 1 wherein the dosageof the implanted splitting ions is in the range of 10¹⁶ cm⁻² to 2×10¹⁷cm⁻², and the energy of the implanted splitting ions is in less than 300keV.
 6. A method as in claim 1 further comprising an additionalimplantation of the SiGe layer with a species of ions selected from thegroup of ions consisting of boron ions, helium ions and silicon ions,with the dosage range of 10¹² to 10¹⁵ cm⁻², and wherein the implantationtakes place before or after the splitting ion implantation.
 7. A methodas in claim 1 wherein the thickness of the epitaxial silicon layer is inthe range of 10 nm to 100 nm.
 8. A method as in claim 1 whereintransferring the epitaxial silicon/SiGe multilayer to an insulatorsubstrate comprises bonding the epitaxial silicon layer surface ofsilicon substrate to an insulator layer on a second substrate to form abonded composite substrate; and thermally annealing the compositesubstrate to split the epitaxial silicon/SiGe multilayer along thedefect zone.
 9. A method as in claim 1 wherein preparing a relaxed SiGelayer directly on the silicon substrate comprises depositing a SiGelayer directly on the silicon substrate whereby a silicon/SiGe interfaceis formed; implanting relaxing ions into the SiGe layer; and annealingto convert the SiGe layer to a relaxed SiGe layer, having a thickness ofbetween about 250 nm to 350 nm.
 10. A method of forming a strainedsilicon layer on an insulator substrate comprising providing a siliconsubstrate; preparing a relaxed SiGe layer to a thickness of betweenabout 250 nm to 350 nm directly on the silicon substrate whereby asilicon/SiGe interface is formed between the silicon substrate and therelaxed SiGe layer; depositing an epitaxial silicon layer to a thicknessof between about 10 nm to 30 nm directly on the relaxed SiGe layer;implanting splitting ions into the epitaxial silicon/SiGe multilayer toa depth of between about 100 nm to 500 nm into the silicon substrate togenerate a defect zone below the silicon/SiGe interface; bonding theepitaxial silicon layer surface to an insulator layer on a secondsubstrate to form a bonded composite substrate; low temperatureannealing the bonded composite substrate to split the epitaxialsilicon/SiGe multilayer along the defect zone at a temperature below400° C.; dry etching to remove a portion of the silicon substrate and aportion of the SiGe layer; high temperature annealing the compositesubstrate to improve the bonding at temperature above 400° C.; andetching the remaining SiGe layer by a process taken from the group ofprocesses consisting of a polishing process, a selective wet etchprocess, and by a combination of polishing and selective wet etchprocesses.
 11. A method as in claim 10 wherein the splitting ionimplantation is performed by plasma immersion ion implantation.
 12. Amethod as in claim 10 wherein the implanted splitting ions comprisemolecular hydrogen ions (H₂ ³⁰).
 13. A method as in claim 10 wherein theimplanted splitting ions are taken from the group of ions consisting ofhydrogen atom ions (H⁺), helium ions, boron ions, silicon ions, argonions, and any combinations thereof.
 14. A method as in claim 10 furthercomprising an additional implantation of the SiGe layer with a speciesof ions selected from the group of ions consisting of boron ions, heliumions and silicon ions, with the dosage range of 10¹² to 10¹⁵ cm⁻², andwherein the implantation takes place before or after the splitting ionimplantation.
 15. A method as in claim 10 wherein the low temperatureannealing time is less than 4 hours.
 16. A method as in claim 10 whereinthe high temperature annealing time is less than 1 hour.
 17. A method asin claim 10 wherein the dosage of the implanted splitting ions is in therange of 10¹⁶ cm⁻² to 2×10¹⁷ cm⁻², and wherein the energy of theimplanted splitting ions is less than 300 keV.
 18. A method as in claim10 wherein preparing a relaxed SiGe layer directly on the siliconsubstrate comprises depositing a SiGe layer directly on the siliconsubstrate whereby a silicon/SiGe interface is formed; implantingrelaxing ions into the SiGe layer; and annealing to convert the SiGelayer to a relaxed SiGe layer.